Positron Emission Tomography-Computed Tomography (“PET/CT”) is a medical imaging technique that combines Positron Emission Tomography (“PET”) and x-ray Computed Tomography (“CT”). Images acquired from both types of systems can be taken in the same session and combined into a single superposed (co-registered) image. Functional imaging obtained by PET scanning, which depicts the spatial distribution of metabolic or biochemical activity in the body, can be aligned or correlated with anatomic imaging obtained by CT scanning
PET/CT has become an important tool to assess the response to therapy for cancer patients. However, respiratory motion can have a major degrading impact on PET-based tumor quantification and delineation. For example, respiratory motion can lead to a tracer concentration underestimation of 30% or more, and overestimation of tumor volume by a factor of two or more. To correct for respiratory motion, the most widely used method is respiratory-gated PET/CT, which divides PET data into different gates based on either temporal phase or respiratory displacement information with potential four-dimensional CT for phase-matched attenuation correction. However, since each gated image contains only a fraction of the detected coincidence events, the increased image noise can lead to substantial overestimation of tracer concentration measured by maximum standardized uptake value (SUVmax).
Another category of motion correction methods utilizes all the detected coincident events, leading to no increase in image noise compared to the static ungated PET image. These methods typically start with respiratory-gated PET or CT data and incorporate estimated image-based motion vectors either into the image reconstruction or postprocessing. The image-based motion vector used in these methods can be derived either from respiratory-gated PET or CT images. If estimated from gated PET images, the motion vectors are subject to the high levels of image noise, and the estimation errors can propagate into the motion-corrected images. On the other hand, gated CT images have much lower noise and can potentially generate more accurate motion vectors, but the patient motion during CT acquisition can be very different from the motion during PET acquisition because of respiration variations. In addition, these approaches may require nonrigid volumetric image registration, which is sensitive to numerous free parameters and typically does not preserve PET tracer concentration. Further, these approaches do not correct for intra-gate motion due to inter-cycle and intra-cycle breathing variation. Alternatives to gating are breath-hold PET/CT methods, which require patients to hold their breath repeatedly during the PET and/or CT acquisition. The breath-hold PET/CT images have the potential for less respiratory motion-blurring effects and more accurately aligned PET/CT images. However, this method is difficult to universally apply, as many patients (e.g., patients with lung cancer) are unable to tolerate holding their breath during treatment.